Electromechanical switching devices such as relays and solid-state electronic switching devices such as power semiconductor devices are currently known and used in vehicle electrical systems. Switching devices such as relays and power semiconductors are regularly used for circuit isolation and protection, switching, and amplification in automotive applications.
Relays have traditionally been used in automotive applications due to their robustness and ability to handle higher currents than early solid-state devices. Relays are electromechanical devices and contain both mechanical and electrical elements. A relay operates by passing a current through a wire coil typically wound around an iron core to create a magnetic field. The magnetic field produced by the coil interacts with a magnetic armature to move the armature from one or more electrical contact positions to one or more different contact positions to either complete or break an electrical circuit depending on the relay design. A relay switching between two contacts operates in the same manner as a simple electrical switch. When current to the coil is switched off, the coil de-energizes and the magnetic field is eliminated. A spring under tension typically uses spring force to return the armature to its original position thereby completing or breaking the circuit depending on the relay design. However, a relay may be designed so that the force of gravity returns the armature to its original position.
Generally the current applied to the coil to produce the magnetic field which actuates the armature is much lower than the current flowing through the contacts when the armature completes the circuit.
Because relays contain moving parts, relays are prone to mechanical wear over their life. The mechanical wear may cause the relay to ultimately fail. Though simple in nature, relays are typically larger and more complex than semiconductor switches. Relay size, mechanical wear, cost and ease of manufacture play a significant role in vehicle electrical system design.
Solid-state switching devices (also called solid-state relays or SSRs) are semiconductors such as transistors and power semiconductor devices that provide a similar switching functionality of an electromechanical relay. In contrast to an electromechanical relay, SSRs contain no moving parts. The absence of moving parts increases a SSRs life and reliability.
In a simple SSR, a semiconductor device such as a transistor contains three or more terminals. An electric current applied to one pair of transistor terminals varies the current flowing through another pair of transistor terminals. Thus, a low current may be applied to a transistor to produce a high current flowing through another set of transistor terminals. In this way, a low current applied at the transistor input acts like an electrical switch to turn on a higher current at the transistor output.
Presently, SSRs can handle higher currents like electromechanical relays. SSRs are smaller and easier to manufacture than electromechanical relays. While SSRs may have a greater lifecycle and are more reliable than electromechanical relays, transient interference caused by sources of electromagnetic interference may falsely trigger an SSR to turn on.
Both semiconductor switching devices and electromechanical relays may malfunction or fail if subjected to electrical currents in excess of their rated current level (overcurrent) or their rated temperature level (overheating). Overcurrent conditions in a switching device may produce heat which in turn may also cause the switching device to overheat.
The proliferation of vehicle electrical systems continues to increase with technological advances in the automotive industry, consumer demand, and regulatory mandates. However, such an increase in vehicle electrical systems requires an increase in electrical components required to operate these systems. Thus, semiconductor switching devices are increasingly being used in automotive applications over electromechanical relays. While electromechanical relays are still widely used in automotive applications, their size, cost and ease of manufacture, compared with semiconductor switches, may be prohibitive in electrical system designs requiring many switching devices.
Currently, automotive switching devices are clustered together in one or more housings and may be segregated by switch type. Typically the housing may be named a relay box, junction box, junction block, power distribution center or power integrator depending on the switch type. The housing or housings are usually located in the vehicle engine compartment.
In instances where a semiconductor switching device fails, the test engineer or technician may be able to isolate the electrical system which has malfunctioned but not the switching device itself. To identify the switching device malfunction, a test engineer or technician must remove the semiconductor switch housing from the vehicle and isolate the malfunctioned switch with test equipment. With the increasing number of semiconductor switching devices used in vehicle electrical systems, the problem of semiconductor malfunction detection and isolation is becoming more burdensome, costly, and time consuming. Accordingly, it is desirable to have a cost-effective system and method for the real-time monitoring of the semiconductor switching devices that will quickly and easily diagnose semiconductor switching device malfunctions in a vehicle.